Biological reactors find increasing use in many areas of industry, including waste treatment plants. Efforts to protect the environment include advanced biological treatment of wastewater through the use of biological reactors, and in particular, fluidized-bed bioreactors. It is the activity of biologically active materials (or “biomass”) within the biological reactor that degrades contaminants in the influent to effect a filtration process. As the biomass treats, through enzymatic reaction, these contaminants, the biomass grows through reproduction within the system. Typically, this activity occurs within a treatment vessel which contains media or other substrate material or carriers on which the biomass attaches and grows as contaminants are consumed. Typical media would include plastic beads, resin beads, sand, activated carbon, or ion exchange resins, among other carriers.
Conventional fluidized-bed bioreactors, such as well-mixed suspended carrier reactors (SCRs), suffer from operational drawbacks in that the media or carriers of the fluidized bed may be subject to excessive buildup of biomass and precipitates, thereby causing compromised flow distribution, excessive media and/or biomass carryover, crusting, increased clogging of filters, and the like. If not properly limited, biomass and precipitate buildup is detrimental to system performance. Uncontrolled biomass film growth in a fluidized bed biological reactor can also result in an undesirable loss of media.
Media bed expansion can, under certain circumstances, be limited by the application of shear at the top of the media bed, but the success of such a control strategy depends upon whether excess biomass and suspended solids can be transported to the top of the fluidized bed. More specifically, it is recognized that such transportation of excess biomass and suspended solids toward the top of the bed is promoted by several dominant mechanisms. For example, media grains that are coated with thicker layers of biomass tend to have an overall particle density that is less than the average particle density within the fluidized bed. Those particles, therefore, are transported to the top of the fluidized bed by virtue of upward moving fluid flow as well as the reduced particle density. This upward movement results in some shear forces acting on biomass-covered particles which does separate some biomass from its supportive media.
Though media bed expansion can therefore be reduced by the application of shear, there remains a need in the industry for an improved system for removing accumulated biomass from a slurry of a fluidized-bed bioreactor to inhibit uncontrolled biomass growth and precipitate accumulation.